Strong gravitational lensing as observed by the Hubble Space Telescope in Abell 1689 indicates the presence of dark matter—enlarge the image to see the lensing arcs.

A gravitational lens is formed when the light from a very distant, bright source (such as a quasar) is "bent" around a massive object (such as a cluster of galaxies) between the source object and the observer. The process is known as gravitational lensing.

Dark matter affects galaxy clusters as well. X-ray measurements of hot intracluster gas
correspond closely to Zwicky's observations of mass-to-light ratios for
large clusters of nearly 10 to 1. Many of the experiments of the Chandra X-ray Observatory use this technique to independently determine the mass of clusters.[32]

The galaxy cluster Abell 2029 is composed of thousands of galaxies enveloped in a cloud of hot gas, and an amount of dark matter equivalent to more than 1014
Suns. At the center of this cluster is an enormous, elliptically shaped
galaxy that is thought to have been formed from the mergers of many
smaller galaxies.[33]
The measured orbital velocities of galaxies within galactic clusters
have been found to be consistent with dark matter observations.

Another important tool for future dark matter observations is gravitational lensing.
Lensing relies on the effects of general relativity to predict masses
without relying on dynamics, and so is a completely independent means of
measuring the dark matter. Strong lensing, the observed distortion of
background galaxies into arcs when the light passes through a
gravitational lens, has been observed around a few distant clusters
including Abell 1689 (pictured right).[34]
By measuring the distortion geometry, the mass of the cluster causing
the phenomena can be obtained. In the dozens of cases where this has
been done, the mass-to-light ratios obtained correspond to the dynamical
dark matter measurements of clusters.[35]

A technique has been developed over the last 10 years called weak gravitational lensing, which looks at minute distortions of galaxies observed in vast galaxy surveys
due to foreground objects through statistical analyses. By examining
the apparent shear deformation of the adjacent background galaxies,
astrophysicists can characterize the mean distribution of dark matter by
statistical means and have found mass-to-light ratios that correspond
to dark matter densities predicted by other large-scale structure
measurements.[36]
The correspondence of the two gravitational lens techniques to other
dark matter measurements has convinced almost all astrophysicists that
dark matter actually exists as a major component of the universe's
composition.

The Bullet Cluster: HST
image with overlays. The total projected mass distribution
reconstructed from strong and weak gravitational lensing is shown in
blue, while the X-ray emitting hot gas observed with Chandra is shown in red.

The most direct observational evidence to date for dark matter is in a system known as the Bullet Cluster. In most regions of the universe, dark matter and visible material are found together,[37]
as expected because of their mutual gravitational attraction. In the
Bullet Cluster, a collision between two galaxy clusters appears to have
caused a separation of dark matter and baryonic matter. X-ray observations show that much of the baryonic matter (in the form of 107–108Kelvin[38] gas, or plasma) in the system is concentrated in the center of the system. Electromagnetic
interactions between passing gas particles caused them to slow down and
settle near the point of impact. However, weak gravitational lensing
observations of the same system show that much of the mass resides
outside of the central region of baryonic gas. Because dark matter does
not interact by electromagnetic forces, it would not have been slowed in
the same way as the X-ray visible gas, so the dark matter components of
the two clusters passed through each other without slowing down
substantially. This accounts for the separation. Unlike the galactic
rotation curves, this evidence for dark matter is independent of the
details of Newtonian gravity, so it is claimed to be direct evidence of the existence of dark matter.[38] Another galaxy cluster, known as the Train Wreck Cluster/Abell
520, appears to have an unusually massive and dark core containing few
of the cluster's galaxies, which presents problems for standard dark
matter models.[39]

This may be explained by the dark core actually being a long,
low-density dark matter filament (containing few galaxies) along the
line of sight, projected onto the cluster core.[40]
The observed behavior of dark matter in clusters constrains whether and how much dark matter scatters off other dark matter particles, quantified as its self-interaction cross section. More simply, the question is whether the dark matter has pressure, and thus can be described as a perfect fluid.[41] The distribution of mass (and thus dark matter) in galaxy clusters has been used to argue both for[42] and against[43]
the existence of significant self-interaction in dark matter.
Specifically, the distribution of dark matter in merging clusters such
as the Bullet Cluster shows that dark matter scatters off other dark
matter particles only very weakly if at all.[44]